Electric systems may need to be tested during a commissioning thereof. Such electric systems may include converter systems comprising inverters which convert DC (direct current) power from a DC power source to AC (alternating current) power. Herein ‘inverter’ generally refers to an electronic device or circuitry that is able to convert direct current to alternating current. An example of the inverter is a semiconductor bridge implemented by means of controllable semiconductor switches, such as IGBTs (Insulated-gate Bipolar Transistor) or FETs (Field-Effect Transistor), which are controlled according to a modulation or control scheme used.
One example of an electric system comprising inverters is a photovoltaic system, such as a photovoltaic power plant, in which photovoltaic panels supply DC power to inverters which convert the DC power to AC power, which may further be supplied to various AC loads via an AC network, for example. Large photovoltaic power plants may comprise a plurality of parallel inverters each receiving DC power from an array of photovoltaic panels.
In commissioning of electric systems, such as a photovoltaic power plant, a connection to the AC network may not be available before the system is commissioned. For example, components of the electric system may need to be tested before the system is allowed to be connected to the AC network, such as a public electrical grid, in order to guarantee that no disturbance will be caused to the AC network upon the connection. However, a proper testing of some components of the electric system may require that an AC voltage with a predetermined magnitude and a predetermined frequency is present on the AC side of the system. For example, the testing of inverters may necessitate that an AC voltage corresponding to that of an AC network that the electric system is to be connected to is present at the AC output of the inverters.
One possible solution to the above problem is to use a separate grid forming generator to generate the required AC voltage and thus to provide a temporary AC network emulating the actual AC network, such as a public electrical grid, for commissioning purposes. Moreover, an artificial load may be used to feed power from the electric system to be tested. However, the use of such separate grid forming generators and artificial loads may cause substantial extra costs and the transportation and setup of such external equipment to possibly remote locations may be time consuming.